The long term goal of this application is to elucidate the mechanisms that sustain functional virus-specific T cells and are responsible for checking viral replication by viruses that establish low-level persistent infection. The duration and magnitude of persistent viral infection govern the survival of memory T cells and their ability to limit viral replication. Thus, persistently infecting viruses that establish high-load (symptomatic) or low-load (often clinically silent) relationships with their hosts have dramatically different effects on the integrity of antiviral memory T cells. A large number of viruses, some with oncogenic potential, asymptomatically infect humans lifelong but may have severely debilitating outcomes in immunocompromised individuals. Polyomaviruses (PyV) are ubiquitous silent persistent pathogens in a variety of vertebrate hosts, including humans, but become opportunistic pathogens in the setting of depressed immune function. Reactivation of human PyV is a leading cause of kidney transplant rejection and, in AIDS patients, causes a fatal brain demyelinating disease. Because the PyV host range is tightly limited to its natural reservoir species, the mouse-PyV model provides the only tractable model to evaluate pathogenesis of this clinically important virus family. In previous funding cycles of this grant, we have utilized mouse PyV infection as a model to investigate antiviral CDS T cell responses to low-level persistently infecting viruses. Because PyV is capable of potent oncogenicity, the PyV-mouse model provides a dual opportunity for understanding mechanisms for immunologic control of persistent infection and virally induced cancer. PyV-specific CDS T cells are visualized in vivo physically using MHC class I tetramers and functionally by viral antigen-induced intracellular cytokine production, cytokine secretion, and cytotoxicity. Using these technologies, we discovered that PyV-specific CDS T cells are newly generated from naive precursors during persistent infection. A guiding theme of this renewal application is that elucidation of the requirements for priming and sustaining these "late-primed" antiviral CDS T cells (Aim 1) will provide insight into developing novel immunotherapeutic strategies for controlling persistent viral infection, with a particular focus on conferring resistance to viral tumorigenesis (Aim 2).